During the previous funding cycle we have: A) Optimized and validated a high-resolution variant of mouse fMRI that can be used longitudinally and in translational human-rodent imaging studies. B) Applied the fMRI variant to investigate a mouse model of Alzheimer's disease (AD) to suggest that the within the hippocampal formation the entorhinal cortex (EC) is the hippocampal subregion differentially vulnerable to AD and A-related toxicity. C) Showied that genetically-modified mice with retromer-deficiency develop hippocampal dysfunction and A accumulation, which together with other studies suggests that retromer-deficiency isolated in the EC of patients is relevant to disease pathogenesis. As a natural extension of the anatomical and molecular findings made during the first cycle of research, two interrelated questions have emerged about the EC in the pathophysiology of AD: A) What are the intracellular mechanisms that account for the vulnerability of the EC, and do they provide insight into general mechanisms of disease? B) Because AD pathology 'spreads' over time, might dysfunction in the EC be linked to dysfunction in other brain regions? As developed in the current proposal, we have relied on a convergence of empirical evidence to generate a 'intracellular hypothesis' that informs the first question, and a 'inter-regional hypothesis' that informs the second. Briefly, the intracellular hypothesis is based on studies showing that, beyond its established role in A production, the retromer also plays a key role in wnt signaling, and that the EC differentially expresses molecules related to the retromer and wnt pathways. Together, we hypothesize a feedback system whereby retromer, A, and wnt are in dynamic equilibrium, which when disrupted can account for key features of the disease. The inter-regional hypothesis is based on lesion studies in non-human primates and on synaptic properties of A that suggest that EC dysfunction can drive dysfunction in other brain regions. The overall goal of this current proposal is to combine mouse fMRI with cellular and molecular techniques to confirm, refine, or refute these testable hypotheses. Although the proposed experiments are hypothesis-driven, in the proposal we also include exploratory studies that can be considered hypothesis- generating, in the case that the hypotheses are completely refuted. Confirming or refining these hypotheses will significantly expand our mechanistic understanding of AD and might suggest novel avenues for therapeutic intervention. We will achieve these goals by completing the following specific aims: